The application of helical screw compressors for supplying compressed air to operate pneumatic construction equipment and for industrial plant compressed air networks and process systems usually requires that the compressor be equipped with some form of compressor throughput or capacity control. Conventional control systems for helical screw air compressors are characterized by controls which operate to throttle the compressor inlet or vent the compressor discharge line during periods of reduced demand. Control systems are known which combine the aforementioned types of controls and which may also include start-stop controls for certain types of motor driven compressors. An example of a compressor control system which combines inlet throttling, discharge line venting, and start-stop control is disclosed in U.S. Pat. No. 3,860,363.
Although helical screw compressors control systems which operate with one or more of the aforementioned types of control may be capable of regulating the supply of compressed gas in accordance with demand, they are relatively inefficient and, in particular, may not be desirable for use with relatively large compressors. Helical screw compressor units which operate with inlet throttling control require considerable power to drive the compressor even when the compressor inlet is throttled to zero compressor gas throughput. Controls which operate to vent the compressor discharge line, and liquid reservoir in the case of liquid injected compressors, may provide for power consumption characteristic which is not much better than inlet throttling control. Power savings for compressor units which operate to vent the compressor discharge line with or without combined inlet throttling are subject to the network storage capacity and the compressed air demand characteristic. Moreover, the start-stop type controls may not be suitable for large electric motor driven compressors.
Helical screw compressors have been modified to include portions of the compressor casing wall which are movable to provide for capacity control by altering the closing of the variable volume chambers formed by the intermeshing rotors. Such devices include axial sliding members such as disclosed in U.S. Pat. No. 3,432,089. The axial slide valve capacity control device is relatively expensive to manufacture and assemble and increases the capital cost of compressor units so equipped. For industrial air and gas compressor units it has been determined that a known type of rotary capacity control valve is less expensive to manufacture and for many compressor applications provides suitable control characteristics for saving compressor input power in accordance with the variations in the demand for compressed air which are normally encountered.